[POWERPC] 83xx: rework platform Kconfig

* Allow multiple boards to be selected in a single build
* Removed Kconfig option '83xx' which existed only for compat with arch/ppc
* Removed Kconfig option 'PPC_MPC836x' since its not used
* Renamed Kconfig option 'MPC834x' to 'PPC_MPC834x' to match others
* Added a multiplatform 83xx defconfig (mpc83xx_defconfig).

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>